Besides determining changes in position of two objects movable relative to each other in a lateral direction, there are measuring tasks where it is necessary, exclusively or possibly additionally, to also determine the distance between these objects in a vertical direction perpendicular thereto. For example, it may be a matter of determining the distance between two plates disposed substantially parallel to each other, these plates being only a small distance from each other. In doing this, interferometric methods as described, for example, in German Published Patent Application No. 10 2007 016 774 and U.S. Pat. No. 4,606,638 present themselves for an extremely precise distance measurement.
The device for determining distance interferometrically described in German Published Patent Application No. 10 2007 016 774 includes an emitter-receiver unit, located on a glass plate, which is placed at a distance to be ascertained from an object, a mirror being disposed on the object. Situated on the glass plate are splitting gratings that split the beam of rays, emitted by the light source, into at least one measuring beam of rays and at least one reference beam of rays. The measuring beam of rays propagates in the direction of the mirror on the object and is thereby reflected back in the direction of the emitter-receiver unit. The reference beam of rays propagates exclusively in the glass plate, and after several reflections in the emitter-receiver unit is superposed interferingly with the measuring beam of rays. From the interference signals thus obtained, the distance between the glass plate and the object or the changes in distance between these components may be ascertained in a conventional manner. However, the device described in German Published Patent Application No. 10 2007 016 774 has the disadvantage that in the event of tilting between the glass plate and the mirror, incorrect distance signals result.
The device described in U.S. Pat. No. 4,606,638 is based on what is termed a Fizeau interferometer, by which the distance between two parallel plates is determinable extremely precisely using an interferometric method. In this case as well, however, for a precise determination of distance, it is essential that the two plates be aligned very exactly in parallel relative to each other. Otherwise, a sharp signal drop results. Thus, the mounting and operational tolerances are extremely small for this device for determining distance interferometrically, as well. Moreover, when using a Fizeau interferometer, measuring accuracy is very limited due to the unavoidable multiple-beam interference.